As you know I like to keep track of my gas milage. The past few weekends I have been traveling and therefore had the chance to get some highway driving instead of the usual commuting. I’ve left the average consumption up on my OBC to monitor my better-than-usual milage and also remind me not to put the pedal to the floor as much as usual, in the hopes of breaking my seemingly unobtainable record of 28mpg. After two fill ups, all the OBC really gives me is false hope. My last fill up, the OBC reported 28.6mpg when calculated miles/gallons was only 24.6. I thought I would give it one more chance in case of extra gas being pumped into the tank so I waited till today’s fill up: OBC reported 29.4 and real milage was 24.5. That’s nearly 5mpg overly optimistic!
Now it is understandable, the OBC’s data most likely comes from the sum of pulse widths of fuel injectors, and fuel measured of a few microliters per pulse, then figure at up to 6500 rpm that is up to 13 thousand injections per minute…so as you can imagine there can easily be a large margin of error! And although I have not watched it as closely, I have always noticed the OBC’s readings being higher than my actual milage, but usually by more like 2 or 3 mpg (and perhaps the upgraded JCW injectors add more to its confusion).
The point is, don’t go assuming you are getting the milage that your OBC is reporting. Not to mention the fact that most of our speedometers read higher speeds then actual; compared with more accurate GPS-determined speeds, mine will read 68 when actual speed is 65. So that means the odometer is reading further than you’ve actually travelled, so actual performance is even worse!
Needless to say I do not advertise my car as a gas-saver. Squeezing 210hp out of a 1.6 liter engine, combined with the high average speeds of Mass’s highways and my hilly stop-and-go commute, my usual milage is probably one of the lowest for MINIs. But it is fun to keep track of and think about gas economy once in a while, especially when prices are this high.
It is apparently possible to calibrate the OBC; I never got round to it on GBMINI#2, and GBMINI#3 is surprisingly close so I don’t need to!
Still averaging 25mpg after 2500 miles …
Comment by Ian C. — May 17, 2005 @ 6:53 pm
I went right out to try that, but my test menu does not have an option 20, it skips right to 21, which resets the software. Interesting.
Comment by Josh Wardell — May 17, 2005 @ 7:20 pm
I call the OBC my ray of optimism, I love how optimisitic it is for mileage and range. On long trips the total of range to go and distance travelled is regularly 400-500 miles and yet I don’t think I’ve ever gotten more than 325 out of a tank. I find the reported mpg to be about 3-4 mpg over what I am actually getting (which is about 27 mpg).
Comment by ukpylot — May 19, 2005 @ 10:26 am
Ooh, forgot to add that I have a picture of my OBC indicating 55mpg which I thought was cool, if wishful thinking.
Comment by ukpylot — May 19, 2005 @ 10:28 am
Josh,
I have kept detail records of all the fills-up on my JCW MCS 210. These records span almost two years, but the JCW 210 was added only last Decemer. I record miles traveled, gallons used, calculated mpg, obc mpg, percent difference, average speed, price per gallon, and total cost.
I have averaged 25.6 calculated mpg but all my obc mpg readings average to 29.2. The obc percent error has ranged from -2.3% to 17% for an average error of 8.8%. I have tried to determine some correlation with average speed but the error % don’t support any relationship.
Interestingly enough, my average mpg prior to December (JCW 210) and after are within about 0.6 mpg! Of course, I have driven slightly more highway miles in the last six months than in the previous months.
I think the constant mpg readout is more valuable than the average mpg as it can help you to stretch a few more miles out of a low tank.
Theo
Comment by Theo — May 22, 2005 @ 2:44 pm